It has become quite common for wireless communication systems to require frequency synthesis in both the receive path circuitry and the transmit path circuitry. Such applications can be found in various communication fields. For example, cellular phone standards in the United States define a cellular telephone system with communication centered in two frequency bands at about 900 MHz and 1800 MHz. For example, U.S. cellular phone standards include (1) the AMPS (analog), IS-54(analog/digital), and IS-95 (analog/digital) standards in the 900 MHz frequency band, and (2) PCS (digital) standards in the 1800 MHz range. A dual-band cellular phone is capable of operating in both the 900 MHz frequency band and the 1800 MHz frequency band.
Recently, wireless local-area networks, or commonly called Wi-Fi, have become popular. For Wi-Fi applications based on the IEEE 802.11 a/b communication standards, a single frequency synthesizer has been used for a dual 802.11 a/b RF transceiver, as described in the aforementioned Related Application. To accommodate signals from both 2.4 GHz and 5 GHz ISM bands, a frequency synthesizer with a wider tuning range is desired.
A frequency synthesizer is conventionally implemented by a phase-locked loop circuit (“PLL”). Reference is now turned to FIG. 1, where a conventional PLL 10 is illustrated. As is well-known to those skilled in the art, PLL 10 will automatically adjust the phase of VCO output signal (“Vout”) 125, and synchronize Vout 125 to reference signal (“Vref”) 105. Frequency divider 130 divides the frequency and phase of Vout 125 by a factor. Phase detector 100 generates phase detector signal Vpd 107, of which the voltage level is proportional to the phase difference between Vref 105 and feedback signal (“Vfb”) 106. Signal Vpd(t) 107 is then filtered by low-pass loop filter 110 having a transfer function F(s), such that the noise and high-frequency components in Vpd(t) are suppressed. The output signal, Vctrl 115 from low-pass loop filter 110 sets the frequency of Vout signals from VCO 120.
However, when the VCO of an RF transceiver is operated in a wider tuning range, it has become more susceptible to errors as a result of process control and temperature variation. As such, it is desirable to provide a frequency synthesizer with self-calibration to compensate for any errors due to process control and temperature variation.
Additionally, conventional VCO designs typically place a VCO's varactor outside of the integrated circuit (“IC”) to take advantage of the varactor's wider and yet linear tuning range. To also accommodate an even wider tuning range, multiple sets of VCOs have been implemented. An example of such multiple VCO design can be found in the frequency synthesizer used in the digital video broadcast system's transceiver. Of course, the trade-off of such multiple VCO design is that the system now is less integrated.
In U.S. Pat. No. 6,388,536, issued to Welland on May 14, 2002, entitled METHOD AND APPARATUS FOR PROVIDING COARSE AND FINE TUNING CONTROL FOR SYNTHESIZING HIGH-FREQUENCY SIGNALS FOR WIRELESS COMMUNICATIONS, a frequency synthesizer is disclosed for use in dual-band, GSM/GPRS cellular phone applications. Such a frequency synthesizer is capable of providing three sets of different frequency synthesis, as well as self-calibration. However, the design is more complicated due to the complex circuitry involved.
Therefore, it is desirable to provide a single set of VCO with an integrated varactor, while achieving self-calibration and fast-locking of any desired frequency.